1. Field
The present disclosure concerns a disc chipper, in particular a knife system therefor, by means of which the position of the cutting edges of individual knives can be adjusted relative to the knife disk.
2. Description of Related Art
Disc chippers are widely used for producing chips to be used for wood pulp production. The knife disc of the disc chipper is provided with knives arranged in an approximately radial direction, said knives chipping the wood against a fixed counterknife. For the part of the chipper, the chip quality is effected by the condition of the counterknife, the bearings of the knife disc, adequate rigidity of the chipper parts, correctly sharpened knives with correctly adjusted width and a small clearance between the knives and the counterknife. In current, relatively large disc chippers, also the planarity of the position of the knife cutting edge is a significant factor for a good chipping result. The planarity means that the knife cutting edges are in the same plane which is perpendicular to the shaft of the chipper. An absolute prerequisite for the successful adjustment of the knife clearance, in other words, the distance between the counterknife and the chipping knives, is a minimum deviation of the knife cutting edges from this plane. The knife clearance of the whole disc is always adjusted according to the knife which is closest to its counterknife.
The positions of the knife cutting edges are based on the manufacturing tolerance of the chipper parts and the correct adjustment of the width of the knives. Because the modern, readily adjustable and exchangeable knife systems of chippers include a multitude of parts, even small manufacturing inaccuracies can cause big differences as they accumulate. The differences even increase, when the parts come from different series of manufacture. Even if the width adjustment has been made carefully, whereby the mutual width difference between knives is in the range+/−0.1 mm, there are differences between the positions of the knife cutting edges when the knives are attached to the knife disc. Thus, the position differences between the knife cutting edges often range between 0.5-1.0 mm and lead, especially with certain wood species, to a prominent formation of filamentous material. Chip quality decreases as the distribution of particle sizes grows. That is, the amount of oversize particles and fines increases. Filaments are formed from the surface wood when the knife clearance is larger than normal. The knife is not able to cut off the tough surface wood, and the strings are carried between the counter knife and the knife to the periphery of the knife disc, and further from there into the chip material.
Precision adjustment of chip length and knife position is disclosed in e.g. Finnish patent 119318 (U.S. Pat. No. 7,735,762). In this method, the position of the knife pressed against the wear plate is adjusted by rotating the relevant wear plate around its hinge. Fine adjustment takes place at the next chip opening by means of an adjustment strip beneath the rear part of the relevant wear plate. The position of the adjustment strip is shifted in a wedge-shaped recess, whereby the rear end of the wear plate rises or falls. Correspondingly, the edge of the knife shifts in the opposite direction. In this method, the position of the knife edge is aligned with sufficient accuracy, that is, into the aforementioned plane which is perpendicular to the shaft. A drawback of this method, however, is that as the position of the wear plate shifts, the reach, i.e. the cut of the following knife changes. The cut refers to the measure from the front face of the rear part of the leading wear plate to the subsequent knife edge, taken in the direction of the disc shaft. The cut defines the resulting chip length. As a consequence of the fine adjustment, the length of the chip produced by the subsequent knife is different from the desired set length. From the point of view of chip quality, a chip length as uniform as possible is important.
Also in U.S. Pat. No. 6,056,030, a disc chipper and an adjustable wear plate are disclosed.